The present invention relates to a method of and apparatus for controlling an internal combustion engine using fuel injection valves and, more particularly, to a method of and apparatus for controlling fuel injection of an engine, which method and apparatus are suitable for a multi-point fuel injection type internal combustion engine.
There is a known control method of controlling a fuel injection valve through detection of engine conditions with various sensors, and repeated calculation of a necessary fuel injection amount with a microcomputer on the basis of detected data of the engine conditions.
The fuel injection amount calculation is based on data representative of engine conditions such as values of the intake air flow rate and the engine speed (r.p.m.) N which are inputted to a microcomputer at prescribed crank angles of the engine, so that calculation is effected in synchronism with engine rotation.
This method has a defect in that calculation cycles or intervals are long when the engine is in an idling condition, so that the control delay is remarkable and a precise calculation of a fuel injection amount can not be effected. Further, the method has a defect in that fuel injection to some cylinders is effected always in specific strokes, such as the suction and expansion strokes, and the other cylinders have fuel injected always in compression and exhaust strokes, for example. Therefore, the mixing condition of the fuel air mixture introduced into the cylinders and the staying time of the mixture in the manifold are different for each cylinder, which causes a difference in the fuel-air mixing degree at each cylinder and a variation in engine characteristics for each cylinder. As a whole, the efficiency and performance of the engine are lowered with the conventional method.
JP-A-55-128630 (1980) discloses an engine control method in which the above-mentioned defects are eliminated. In JP-A-55-128630, fuel is injected at fixed time intervals irrespective of the engine rotation, and the fuel injection amount also is calculated at constant intervals of time. With respect to the fuel injection at fixed time intervals according to the method disclosed in this publication, when the method is used in a multi-point fuel injection system, the fuel injection start point from a suction stroke, for example, is not constant, but changes widely, that is, in some cases fuel is injected at the compression stroke, and in other cases fuel is injected in the suction stroke, for instance. When the fuel is injected in the compression stroke, there is a relatively large time interval between a time at which a fuel injection amount is calculated for fuel injection of the compression stroke and a time at which the fuel amount is sucked into the cylinder, which means that the injected fuel amount is based on relatively old data such as the intake air flow rate and engine speed (r.p.m.). Although, when the engine runs at a low engine speed such as during idling, there is substantially no control delay, in case of a high speed operation of the engine, particularly in case of acceleration of the engine, a sufficiently precise fuel injection amount can not be calculated according to the prior art. Therefore, the calculated air ratio sometimes deviates from the theoretical fuel-air ratio, so that purification of exhaust gas is not sufficient even if a three way catalyst is used, and the engine according to the prior art can not keep to the recent strict regulation of exhaust gas.
There is another method of fuel injection control disclosed in JP-B-62-7380 (published February of 1987), wherein fuel injection valves driven at the same time as each other are driven so as to be forcibly opened for a short fixed time period at a fuel injection time, an optimum fuel injection time is calculated according to engine conditions to establish the opening time of the valves, and then the fuel injection valves are closed after lapse of the time of the short fixed time period and the optimum fuel injection time period. According to this method, in usual operation of the engine, the optimum injection time is calculated on the basis of engine condition data obtained only in the short fixed time, and is not calculated after the short fixed time. Further, according to this method, since the injection valves (the number of which are 4 in four cylinder engine) are driven simultaneously to inject fuel, even if one of the valves is opened at an optimum injection time for that engine cylinder in an engine cycle, the injection time for the others may not be optimum. Summary of the Invention
An object of the present invention is to provide an engine control method and apparatus wherein a precise fuel injection amount for each engine cylinder can be calculated even if there is a large change in intake air flow rate during acceleration or deceleration, whereby emission of exhaust contamination can be strictly restricted.
According to an aspect of the present invention, data for engine conditions, such as engine speed (r.p.m.) and flow rate of air taken in each of a plurality of cylinders of the engine, is detected by various sensors; an amount of fuel to be injected into each engine cylinder is calculated repeatedly on the basis of the detected data at fixed time intervals; the calculated amount of fuel is injected into each engine cylinder, so that the calculated fuel injection amount is injected into each cylinder at an optimum fuel injection time in an engine cycle and the calculated fuel amount can be renewed at least once in a time period from the start of fuel injection until completion of the fuel injection in a load operation of the engine in which the engine load is larger than that which occurs during idling.
According to the aspect of the invention, when the engine is in a condition in which the flow rate of air sucked into the engine cylinder changes, a calculated value of the fuel injection amount can be renewed and the fuel of renewed value is injected at the optimum injection time, so that fuel in an amount very close to that really required for the engine is injected. Therefore, the fuel-air mixture ratio is not significantly different from the theoretical fuel air ratio and the exhaust gas can be purified sufficiently by a catalyst, for example.
According to our experimental investigation, when the time interval of calculation of fuel injection amount is 5 msec or less, the deviation in air-fuel ratio from a theoretical ratio can be kept within a prescribed value in which the exhaust gas can be purified so as to keep to a strict regulation of exhaust gas, although a time interval of 2 msec is more preferable.
The optimum fuel injection time or position is a position which is close to a starting point of an intake stroke in an engine cycle, such that the fuel injected at the position is fully sucked into the cylinder during the intake stroke of this engine cycle.
In a multi-cylinder type engine, an optimum fuel injection period differs for each cylinder, namely, each cylinder has an optimum fuel injection period with respect to an intake stroke of an engine cycle. According to the present invention, fuel in an amount which is calculated on the basis of newest engine condition data in an engine cycle is injected in each cylinder of the engine at the optimum fuel injection time for each cylinder, so that fuel can be combusted at an optimum fuel-air ratio in each cylinder, and so that the exhaust gas can be purified by a suitable catalyst which is known to be high in conversion rate of CO, HC and NOx at a suitable fuel air ratio.